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Math OER
 Shapeshifting: Changing a Number 

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Our first big Math 20 topic is Shapeshifting. This is our nickname for when we do something to a single number.

What can we do with a single number?

Consider a situation in which child rolls a die 1,000 times. An even number is rolled 499 times.

We can write this result in fraction format as 4991,000.

This fraction is very close to one-half. In fact, we would expect that if the child rolled the dice many more times the nearness to one-half would increase. The child has "proved" in his or her mind that the chance rolling an even number is one-half. So it makes sense to round the answer to one-half.

We could change the number one-half to decimal format by writing it as 0.5.

Tangentially, when it is spoken aloud the decimal 0.5 has three possible names. An instructor who is lecturing would use the dictation name and say "zero point five" because saying "zero point..." gives a heads-up to listening ears. People thinking to themselves or working together would probably use the casual name and say "point five". There is also a formal name, "five tenths", created from the place value of the final digit.

We could change the number one-half to percent format by writing it as 50%.

Finally, if the number was a measurement we could change the format of its measurement units. For example, a length of 0.5 feet could also be written as 6 inches.

To summarize, we will explore more deeply how and when we "shapeshift" a number by rounding or by changing formats (fraction, decimal, percent, measurement units).


Rounding

Decimal Accuracy §1.1, §4.1

Very Accurate

Decimals can be very accurate.

gaming mouse High-performance gaming optical mice can measure a movement as small as six ten-thousandths of an inch.

LEGO bricks fit together so well because the molds that make them are even more precise. Their accuracy is within two thousandths of a millimeter.

Let's review the place value names.

1. State the place value of each digit in the number 9876.54321.

from left to right: hundreds, tens, ones, tenths, hundredths, thousandths, ten-thousandths, hundred-thousandths

place value problem

We should also do two problems that check whether zeroes in place values confuse us.

2. Place the following numbers in ascending order (smallest to largest).

2,608 → 2,680 → 2,860 → 20,680

20,680       2,680       2,860       2,608

3. Place the following numbers in ascending order (smallest to largest).

0.57429       0.57491       0.574905       0.57949

0.57429 → 0.574905 → 0.57491 → 0.57949

water person Here is an example in which so many decimal places really matter.

Cole Paviour invented a famous machine that used a computerized motion-capture of an athlete moving to control the timing of more than two thounsand separate water drop dispensers. This machine needs accuracy to the millionths place.

Imagine that one of Cole Paviour's measurements was 5.714286 seconds.

4. Confirm that the measurement 5.714286 seconds is accurate to the millionths place.

Yes. The 7 is in the tenths, the 1 in the hundredths, the 4 in the thousandths, the 2 in the ten-thousandths, the 8 in the hundred-thousandths, and the 6 in the millionths.

Too Accurate?

In the situation above the decimals really were amazingly accurate. However, some decimals look accurate but are not.

The most common source of fake accuracy is division.

Consider three different division stories that use 40 inches ÷ 7 = 5.714286....

5. I am planning my kid's birthday party. There will be six children attending, so I need 7 party favor bags (including the one for my own kid). I have 40 pieces of candy for in the bags. How many pieces go in each bag?

To fairly distribute 40 pieces of candy into 7 party favor bags, each bag gets 5 pieces. I can see 5 pieces ÷ 7 = 35 pieces. Then I eat the remaining 5 pieces myself.

An alert student might notice that this problem did not follow the usual rules for rounding. The tenths place was 5 or more (it was 7) but we still rounded down. That is okay. Real life can be like that.

It is true that 40 inches ÷ 7 = 5.714286... which looks like the number from Example 4. But rounding this answer with any decimal places does not make sense. No one will get a part of a candy. We should just drop all the decimal places.

Notice how the decimal places appeared because of division, not careful measurement. They were fake accuracy.

6. A pharmacist has a small container whose label tells her it contains 40 milligrams of medicine. She divides it into 7 doses. How many milligrams are in each dose?

Since the initial value was only accurate to a whole number of milligrams, we could use the same number of decimal places and say 40 milligrams ÷ 7 ≈ 6 milligrams. Making equal piles might not increase our accuracy beyond what we originally knew. We have no reason to believe the medicine's accurate total weight is precisely 40 milligrams. It could be 39.6 milligrams or 40.4 milligrams, or some other amount that whomever sent it to the pharmacist had rounded. The pharmacist did not re-weigh the medicine before dividing it into doses, knowing that for this medicine that would be a waste of her time. So 6 milligrams is an honest answer.

We could also use one more decimal place and say 40 milligrams ÷ 7 ≈ 5.7 milligrams. The pharmacist's scale can measure accurately to the tenth of a milligram. Making equal piles can often increase our accuracy by one decimal place. That is another honest answer.

But if we said 40 milligrams ÷ 7 ≈ 5.71 milligrams we would be exaggerating. The pharmacist's scale is not accurate to the hundredth of a milligram. No matter how carefully we make equal piles we probably cannot increase our accuracy by two decimal places.

Notice how the decimal places appeared because of division, not careful measurement. But adding one more decimal place could be justified. Making about ten piles could increase our accuracy by a tenth. Also, the pharmacist's scale could measure accurately to the tenth of a milligram.

7. A carpenter had a 40 inch board, and cut it into seven equal lengths. How many inches long is each piece?

If we said 40 inches ÷ 7 ≈ 6 inches we probably insult the carpenter. A tape measure and saw should be accurate to the tenth of an inch.

We should use one more decimal place and say say 40 inches ÷ 7 ≈ 5.7 inches. That is an honest answer.

If we said 40 inches ÷ 7 ≈ 5.71 inches we are probably exaggerating. Maybe the original board was not actually shorter than 40 inches, maybe the carpenter was extremely careful and used his tools to the limit of their accuracy, and maybe the extra wood was discarded unstead of letting the final piece be a smidgen too long. In this situation the tools might justify using two more decimal places in our answer. But those are a lot of maybes.

Notice how the decimal places appeared because of division, not careful measurement. But adding one more decimal place is very justified. Making about ten piles could increase our accuracy by a tenth. Also, the carpenter's tools should measure accurately to the tenth of an inch.

In conclusion, a measurement tool can be very accurate. But when division is the source of a decimal answer, the general guideline is to not add more than one more decimal place than in the original values.

A General Rule for Accuracy

When division is the source of a decimal answer, use the same number of decimal places as in the original values, or perhaps one more decimal place.

Your turn to do an example.

8. Clara cans 15 pints of salsa. There are 16 pints in a gallon. How much of a gallon did she make?

15 pints ÷ 16 pints per gallon = 0.9375 of a gallon ≈ 0.94 of a gallon

We can reliably divide a gallon into 100 pieces, each would be about 2.5 tablespoons. Clara's salsa production is probably not accurate to the tenth of a tablespoon, so we should stop there.

Your Turn to Round

In the examples above the decimal answer was rounded for you. Now it is your turn to round decimal answers!

First, a carefully worded definition.

Rounding

Rounding is ideally reducing the accuracy of an answer to remove fake accuracy, using four steps:

• Pick an appropriate place value

• Make zero the digits to the right (this removes decimal digits)

• Of the digits just made zero, if the largest/leftmost was 5 or more then increase by 1 the digit of the picked place value

• Do not change the digits to the left

No one else defines rounding so carefully. But we can be extra awesome.

Let's unpack this definition.

First, we ideally only round answers with fake accuracy. The exceptions should be few. When someone like Cole Paviour works hard to be genuinely super-accurate it would be a shame to throw away that accuracy for no reason!

Second, we pick an appropriate place value. You were just introduced to the relevant issues. How accurate are the situation's initial values? How accurate are the tools being used? Is the situation one of the rare cases when making equal sized piles can add not merely one but two decimal places of accuracy?

(In general, students worry much more about this decision than math instructors. We want you to learn math, not spend time agonizing over whether a teaspoon is accurate to the hundredth or thousandth of a pint.)

Third, notice that we are making digits zero. It can be tempted to use sloppy thinking and imagine that we are "removing" decimal place digits. But truthfully they are still there. We have only set them to zero, which means we can skip writing them.

Time for many more examples!

9. Round each number to the nearest... ones?      tens?      hundreds?      thousands?

(a) 4,573 to the ones, 4,570 to the tens, 4,600 to the hundreds, 5,000 to the thousands

(b) 625 to the ones, 620 to the tens, 600 to the hundreds, 1,000 to the thousands

(c) 17,349 to the ones, 17,350 to the tens, 17,300 to the hundreds, 17,000 to the thousands

10. Round each number to the nearest... tenth?      hundredth?      thousandth?      ten-thousandth?

(a) 2.0 to the tenth, 1.95 to the hundredth, 1.953 to the thousandth, 1.9528 to the ten-thousandth

(b) 3.9 to the tenth, 3.85 to the hundredth, 3.853 to the thousandth, 3.8526 to the ten-thousandth

(c) 0.1 to the tenth, 0.07 to the hundredth, 0.072 to the thousandth, 0.0725 to the ten-thousandth

(d) 25.8 to the tenth, 25.79 to the hundredth, 25.790 to the thousandth, 25.7901 to the ten-thousandth

(e) 0.7 to the tenth, 0.67 to the hundredth, 0.667 to the thousandth, 0.6667 to the ten-thousandth

(f) 1.3 to the tenths, 1.27 to the hundredths, 1.273 to the thousandths, 1.2727 to the ten-thousandths

(g) 5.7 to the tenth, 5.75 to the hundredth, 5.750 to the thousandth, 5.7495 to the ten-thousandth

In problem 10d please notice the unexpected thing when rounding to the nearest thousandth. When we are asked to write to the thousandths place, we must write three decimal digits. We write a zero to "fill up" to the thousandths place.

11. Find the rounding error below, where someone tried to round 478.3469 to different place values.

rounding problem

The hundreds digit is less than five, so when rounding to the nearest thousand we get 0 instead of 1,000.

Our definition of rounding warned us to not round in the middle of a problem. If we round too early we introduce error. Let's consider a few examples.

12a. In Eugene it rains, on average, about 1.15 inches per week. How many inches of rain is this per year? Round correctly, only rounding the answer.

(a) 1.15 inches × 52 weeks = 59.8 inches

12b. In Eugene it rains, on average, about 1.15 inches per week. How many inches of rain is this per year? Round incorrectly, doing 1 inch × 52 weeks.

(b) 1 inch × 52 weeks = 52 inches

Soon we will have the tools to see that rounding too early in the problem made the answer about 13% too small. That might matter a lot if we were farmers budgeting for our annual water bill.

Here is another example of rounding too early.

13a. You do assistant contractor work that earns a little over $1,000 per month after taxes. You work for 3 months and 1 week, then have time between jobs, then work for 5 months and 1 week. How much do you earn total? Round correctly, only rounding the answer.

(a) (3.25 + 5.25) × $8,500

13b. You do assistant contractor work that earns a little over $1,000 per month after taxes. You work for 3 months and 1 week, then have time between jobs, then work for 5 months and 1 week. How much do you earn total? Round incorrectly, doing (3 + 5) × $1,000.

(b) (3 + 5) × $8,000

Soon we will have the tools to see that rounding too early in the problem made the answer about 6% too small. That could be a big deal if we were planning our personal budget.

As a concluding exercise, work with a partner to invent a word problem. Solve it accurately, doing any rounding at the end. Then round too early, and see how much the answer changes. Try to invent a problem for which rounding early would be utterly disastrous.

videos

Video example problems have three images to click on. You can see a video step-by-step answer, a written step-by-step answer, or only the answer. If you find yet more helpful videos, please let your instructor know so that this website can be updated and improved!

Kahn Academy

Introduction to Place Value

Finding Place Value

The Organic Chemistry Tutor

Rounding Numbers and Rounding Decimals - The Easy Way!

YouTube Problems

play video show steps show answer Round 34,528 to the nearest thousand. Identify the requested place value: 34,527
Also consider the place value immediately to the right: 34,528
Is the 5 as big as 5 or more? Yes, so it makes us increase the 4 before we zero out that 5 and everything to its right: 35,000
35,000

play video show steps show answer Round 34,528 to the nearest ten. Identify the requested place value: 34,528
Also consider the place value immediately to the right: 34,528
Is the 8 as big as 5 or more? Yes, so it makes us increase the 2 before we zero out that 8 (there is nothing to its right): 35,530
35,530

play video show steps show answer Round 34,528 to the nearest hundred. Identify the requested place value: 34,528
Also consider the place value immediately to the right: 34,528
Is the 2 as big as 5 or more? No, so leave the 5 unchanged as we zero out that 2 and everything to its right: 34,500
34,500

play video show steps show answer Round 5.6783 to the nearest one. Identify the requested place value: 5.6783
Also consider the place value immediately to the right: 5.6783
Is the 6 as big as 5 or more? Yes, so it makes us increase the 5 before we zero out that 6 and everything to its right: 6
6

play video show steps show answer Round 5.6783 to the nearest hundredth. Identify the requested place value: 5.6783
Also consider the place value immediately to the right: 5.6783
Is the 8 as big as 5 or more? Yes, so it makes us increase the 7 before we zero out that 8 and everything to its right: 5.68
5.68

play video show steps show answer Round 5.6783 to the nearest thousandth. Identify the requested place value: 5.6783
Also consider the place value immediately to the right: 5.6783
Is the 3 as big as 5 or more? No, so leave the 8 unchanged as we zero out that 3 (there is nothing to its right): 5.678
5.678

play video show steps show answer Round 5.6783 to the nearest tenth. Identify the requested place value: 5.6783
Also consider the place value immediately to the right: 5.6773
Is the 7 as big as 5 or more? Yes, so it makes us increase the 6 before we zero out that 7 and everything to its right: 5.7
5.7

Textbook cover

Textbook Exercises for Decimal Accuracy

This list of recommended odd-numbered textbook problems is designed for a hypothetical student with a "typical" math background. If your math foundation is weak, do even more odd-numbered problems. If your math foundation is strong, do fewer.

Please read the advice on doing homework in the study skills page.

Section 1.1 (page 9) # 13, 15, 17, 19, 67, 71

Section 4.1 (page 197) # 17, 19, 21, 27, 29, 31, 33, 35, 49, 55, 57, 59, 69, 71

Estimating §1.3, §4.1

As we have seen, rounding early can drastically change an answer. But it can be a very helpful thing to do as a quick "check" before we attempt a problem.

Definition

Rounding at the very beginning of a problem is called estimating.

For now, use estimating to quickly get a rough idea how large the answer should be. This an artificial but useful reason to estimate.

After we estimate, we could actually solve the problem. Is our answer close to the estimated amount? If not, we might have made a careless mistake (on paper or with the calculator) and can go back to find what went wrong.

14. Estimate the answer to $47,900 ÷ 12 by rounding both numbers to the nearest ten, before using a calculator to do the problem.

$47,900 ÷ 10 = $4,790, which is not very close to the actual calculation of $47,900 ÷ 12 = $3,991.67

Many real life situations force us to estimate. For example, if I am planning a road trip the price of gasoline will not be the same at every gas station. To budget for my trip I will pick one value to use as my "typical" estimated price.

For that reason we will (later on) do math in situations that start with estimation and have no precise "right answer" to afterwards compare.

15. Estimate the sum or difference. Round all numbers (you pick how much to round) before mentally adding or subtracting.

Note: answers will be different if the amount rounded is different.

(a) 60 + 30 = 90

(b) 350 − 90 = 260

(c) 120 + 80 = 200

(d) 460 − 180 = 280

(e) 6,000 + 100 = 6,100

(f) 8.15 − 0.01 = 8.14

(g) 3,500 + 6,300 = 9,800

(h) 1.675 − 1 = 0.675

16. Estimate the product or quotient. Round all numbers (you pick how much to round) before mentally multiplying or dividing.

Note: answers will be different if the amount rounded is different.

(a) 20 × 30 = 600

(b) 930 ÷ 10 = 93

(c) 50 × 5 = 250

(d) 2,000 ÷ 200 = 10

(e) 870 × 10 = 8,700

(f) 460 ÷ 20 = 23

(g) 0.01 × 0.01 = 0.0001

(h) 200 ÷ 200 = 1

Here are more estimation problems.

17. This week the national average price for a certain item was $3.025, which is an increase of $0.284 from last week. First estimate last week's price. Then solve to find the actual value of last week's price.

estimate $3.025 − $0.284 ≈ $3.0 − $0.3 = $2.70

actual $3.025 − $0.284 = $2.741

18. A square has four equal sides of length 2.48 feet. The perimeter of a square is the sum of its four sides. First estimate the square's perimeter. Then solve to find the actual value of the square's perimeter.

estimate 2.48 × 4 ≈ 2.5 × 4 = 10 feet

actual 2.48 × 4 = 9.92 feet

19. Circle the correct response. If each number in a sum is rounded up, the estimated total

will be an overestimate    /    will be an underestimate    /    could be either an overestimate or underestimate

will be an overestimate

20. Circle the correct response. If each number in a sum is rounded down, the estimated total

will be an overestimate    /    will be an underestimate    /    could be either an overestimate or underestimate

will be an underestimate

21. Circle the correct response. If some of the numbers in a sum are rounded up and others are rounded down, the estimated total

will be an overestimate    /    will be an underestimate    /    could be either an overestimate or underestimate

could be either an overestimate or underestimate

videos

Video example problems have three images to click on. You can see a video step-by-step answer, a written step-by-step answer, or only the answer. If you find yet more helpful videos, please let your instructor know so that this website can be updated and improved!

YouTube Problems

play video show steps show answer Estimate 4,872 + 1,691 + 777 + 6,124 by first rounding each number to the nearest thousand. ≈ 5,000 + 2,000 + 1,000 + 6,000 = 14,000 14,000

play video show steps show answer Estimate the sum 23,649 + 54,746 by first rounding to the nearest hundred. ≈ 23,600 + 54,700 = 78,300 78,300

play video show steps show answer Estimate the difference 54,751 − 23,649 by first rounding to the nearest hundred. ≈ 54,800 − 23,600 = 31,200 31,200

play video show steps show answer Estimate the product 824 × 489 by first rounding to the nearest hundred. ≈ 800 × 500 = 400,000 400,000

play video show steps show answer Estimate the product 8.91 × 22.457 by first rounding to the nearest one. ≈ 9 × 22 = 198 198

play video show steps show answer Estimate the quotient 78.2209 ÷ 16.09 by first rounding to the nearest ten. ≈ 80 ÷ 20 = 4 4

Textbook cover

Textbook Exercises for Estimating

This list of recommended odd-numbered textbook problems is designed for a hypothetical student with a "typical" math background. If your math foundation is weak, do even more odd-numbered problems. If your math foundation is strong, do fewer.

Please read the advice on doing homework in the study skills page.

Section 1.3 (page 31) # 41, 43, 45, 47, 59, 61, 63

Section 1.6 (page 74) # 35, 37, 39, 41, 43, 45

Congratulations! You are at the end of the first subtopic (Rounding) for our first big topic (Shapeshifting).

You have now thought more carefully than most people about decimals, their accuracy, when to round, and how much to round.

Ten Exercises

Try these ten exercises on scratch paper. Work in a study group if you can! Notice where your notes need improvement. After you are very happy with your answers, you can use this form to ask me to check your work. Can you get at least 8 out of 10 correct?

1. What number is two hundred twelve thousandths?

2. Which is the largest decimal? 0.243, 0.423, 0.324, 0.0423, 0.4023

3. Which is the smallest decimal? 0.243, 0.423, 0.324, 0.0423, 0.4023

4. Round 17,398.967 to the nearest ten.

5. Round 98,807.728 to the nearest tenth.

6. Round 54,498.66118 to the nearest thousand.

7. Notice that 2 ÷ 11 = 0.181818... Round that decimal to the nearest thousandth.

8. Alicia's rectangular garden bed has two long sides of length 2.74 meters and two short sides of length 1.22 meters. Estimate the sum of all four sides.

9. Ben's rectangular garden bed has two long sides of length 3.048 meters and two short sides of length 0.9144 meters. Estimate the sum of all four sides.

10. Which garden bed estimate will be closer to the actual sum? How can you know without calculating the actual sums?

Random Exercises

Try these exercises on scratch paper. Work in a study group if you can! Notice where your notes need improvement. Check your work when you are done.

Decimal Accuracy

Estimating


Fraction Format

Thinking About Division §1.6, §2.1, §2.5

There are two ways to think about division. Unfortunately, most people are only taught one way and this causes people to get stuck.

Dealing Out Cards

The first way to think about division is dealing out cards.

If I want to model 6 ÷ 3 = 2 then I act out having six cards, and dealing them out to three people until I am done.

How does it work? What question do I ask to find the answer? (An answer we know is 2.)

Division: Dealing Out Cards (Works Great!)

Model 6 ÷ 3 = 2 by dealing out cards.

6 becomes the number of cards total.

3 becomes the number of piles.

2 becomes the number of cards per pile.

Unfortunately, this way of thinking does not help with dividing by fractions.

Division: Dealing Out Cards (Has Problems!)

Deal out cards to model 6 ÷ 12 = 12.

We get stuck! How do we deal cards to half a pile? What is "half a pile" anyway?

Making Piles of a Fixed Size

The second way to think about division is making piles of a fixed size.

If I want to model 6 ÷ 3 = 2 then I act out having six cards, and dealing them out them out into piles of size 3 until I am done.

How does it work? What question do I ask to find the answer? (An answer we know is 2.)

Division: Piles of Fixed Size (Works Great!)

Model 6 ÷ 3 = 2 by making piles of a fixed size.

6 becomes the number of cards total.

3 becomes the number of cards per pile.

2 becomes the number of piles.

This way of thinking does help us think about dividing by fractions.

Division: Piles of Fixed Size (Still Works Great!)

Model 6 ÷ 12 = 12 by making piles of a fixed size.

6 becomes the number of cards total.

12 becomes the number of cards per pile. (We must rip cards in half!)

12 becomes the number of piles.

The second way of thinking also allows us to understand why division by zero, which is undefined, is in a few situations treated as if the answer is infinity. If I had some cards and tried to make piles of size zero I can do this easily. I just never stop!

Your turn to think carefully about division.

22. Solve each equation by multiplying or dividing, if possible.

(a) 28

(b) 28

(c) 0

(d) undefined

(e) 10,000

(f) 1

23. 4 ÷ 12 =

Imagine having 4 pieces of paper. We rip each in half while setting those halves down as "piles". We get 8 piles.

So 4 ÷ 12 = 8

24. 12 ÷ 14 =

Imagine having half a piece of paper. We rip it in half again, making quarters, while setting those quarters down as "piles". We get 2 piles.

So 12 ÷ 14 = 2

videos

Video example problems have three images to click on. You can see a video step-by-step answer, a written step-by-step answer, or only the answer. If you find yet more helpful videos, please let your instructor know so that this website can be updated and improved!

Kahn Academy

The Idea of Division

YouTube Problems

(we need some problems here)

Textbook cover

Textbook Exercises for Thinking About Division

This list of recommended odd-numbered textbook problems is designed for a hypothetical student with a "typical" math background. If your math foundation is weak, do even more odd-numbered problems. If your math foundation is strong, do fewer.

Please read the advice on doing homework in the study skills page.

Section 1.6 (page 76) # 77, 79, 81

Section 2.1 (page 102) # 75, 77, 81, 83

Section 2.5 (page 139) # 55, 59

Divisibility and Factors §2.3

Visualizing Factors

Our next step in thinking carefully about division is to visualize factors.

Definition

Factors are the numbers you multiply to get another number.

That is a slightly sloppy definition, but it is good enough.

We can look at a multiplication equation to find factors.

Examples of Factors

3 and 5 are two of the factors of 15 because 3 × 5 = 15

1 and 15 are two more factors of 15 because 1 × 15 = 15

Some students find it helpful to think about factors by imagining all the ways to make a rectangle with blocks or coins.

factors example

Note that 1 and the number itself are always factors!

Divisibility Rules

How would we find factors if we do not have multiplication equations handed to us?

We could just think hard, and maybe guess-and check.

Does 6 Work?

Is 6 a factor of 96?

Um...let me think...yes, because 6 × 16 = 96

But some tricks can help us.

Here are divisibility rules you already know:

Here are divisibility rules that might be new to you:

There are yet more divisibility rules, but these are the ones that are easy to use.

Let's apply the divisibility rules to 13,512.

25. Is 13,512 divisible by... 2?       3?       4?       5?       6?       9?

Divisible by 2? Yes, because the one's place value digit is 2, which is even.

Divisible by 3? Yes, because the sum of digits is 1 + 3 + 5 + 1 + 2 = 12 and three goes into 12.

Divisible by 4? Yes, because two-digit number formed from its ten's and one's digits is 12 and four goes into 12.

Divisible by 5? No, because the one's place value digit is not zero or five.

Divisible by 6? Yes, because it was divisible by both 2 and 3.

Divisible by 9? No, because the sum of digits is 1 + 3 + 5 + 1 + 2 = 12 and nine does not go into 12.

Let's also apply the divisibility rules to 2,016.

26. Is 5,661 divisible by... 2?       3?       4?       5?       6?       9?

Divisible by 2? No, because the one's place value digit is 1, which is odd.

Divisible by 3? Yes, because the sum of digits is 8 + 6 + 6 + 1 = 18 and three goes into 18.

Divisible by 4? No, because two-digit number formed from its ten's and one's digits is 61 and four does not go into 61.

Divisible by 5? No, because the one's place value digit is not zero or five.

Divisible by 6? No, because it was not divisible by both 2 and 3.

Divisible by 9? Yes, because the sum of digits is 8 + 6 + 6 + 1 = 18 and nine goes into 18.

27. Is 5,025 divisible by... 2?       3?       4?       5?       6?       9?

Divisible by 2? No, because the one's place value digit is 5, which is odd.

Divisible by 3? Yes, because the sum of digits is 2 + 2 + 5 = 12 and three goes into 12.

Divisible by 4? No, because two-digit number formed from its ten's and one's digits is 25 and four does not go into 25.

Divisible by 5? Yes, because the one's place value digit is a zero or five.

Divisible by 6? No, because it was not divisible by both 2 and 3.

Divisible by 9? No, because the sum of digits is 1 + 3 + 5 + 1 + 2 = 12 and nine does not go into 12.

28. Is 73,080 divisible by... 2?       3?       4?       5?       6?       9?

Divisible by 2? Yes, because the one's place value digit is 0, which is even.

Divisible by 3? Yes, because the sum of digits is 7 + 3 + 8 = 18 and three goes into 18.

Divisible by 4? Yes, because two-digit number formed from its ten's and one's digits is 80 and four goes into 80.

Divisible by 5? Yes, because the one's place value digit is a zero or five.

Divisible by 6? Yes, because it was divisible by both 2 and 3.

Divisible by 9? Yes, because the sum of digits is 7 + 3 + 8 = 18 and nine goes into 18.

These divisibility rules let us quickly find some factors.

Let's play the Factor Game! Here is a copy of the game board to use with an in-class demonstration.

factor game

29. factor game problem Xavier and Odette are playing the Factor Game. Xavier is X's. Odette is O's. The board is shown below. It is Xavier's turn.

(a) If Xavier picks 15 then Odette cannot reply and he comes out ahead by 15 points.

(b) She picks 28 and Xavier replies with 14.

(c) The next turn Xavier would be able to pick 28 and she would be unable to reply.

Being able to find factors quickly will soon allow us to do fraction arithmetic more easily. But we have a bit more preparation to do.

There are two flavors of thorough factor finding. In some situations we want to find all the factors. In other situations we want to find the prime factors.

Finding all the factors will be useful when reducing a fraction.

Finding prime factors will useful when finding a common denominator for fractions.

Ready?

Finding All the Factors

We find all the factors of a number by making a two-column list. Count 1, 2, 3,... in the first column. List any matching factors in the second column. When the columns get to the same value we can stop.

30. Find all the factors of 66.

This time our first column counts up from 1 to 10. The, the next value for the first column would be 11, which is already listed in the second column. So we can stop.

1       66
2       33
3       22
4       not a factor
5       not a factor
6       11
7       not a factor
8       not a factor
9       not a factor
10      not a factor

So 66 has eight factors: 1, 2, 3, 6, 11, 22, 33, 66

31. Find all the factors of 24.

24 has eight factors: 1, 2, 3, 4, 6, 8, 12, 24

32. Find all the factors of 60.

60 has twelve factors: 1, 2, 3, 4, 5, 6, 10, 12, 15, 20, 30, 60

33. Which has more factors, 36 or 40?

36 has nine factors (more): 1, 2, 3, 4, 6, 9, 12, 18, 36

40 has eight factors: 1, 2, 4, 5, 8, 10, 20, 40

34. Of all the numbers we just considered, only 36 had an odd number of factors. What happened to make 36 special?

36 = 6 × 6. When we made those two columns, the bottom number for both columns was the same, 6. So even though factors come in pairs, there is a special case where the "bottom" pair of factors is the same number repeated.

Finding Prime Factors

We find prime factors by making a factor tree and noting the "leaves".

Remember factor trees?

Here is one factor tree for 48.

rounding example

35. Write a different factor tree for 48 that does not start with 6 and 8.

Answers will vary.

We could start with 2 × 24 or with 3 × 16 or with 4 × 12.

We could also start by splitting 48 into three factors, such as 2 × 2 × 12 or 2 × 3 × 8.

Either circle or "bring down" the leaves of your factor tree so you do not make a careless mistake and forget any of them when writing your answer.

To be polite, list the prime factors in order. Write them as a product (separated by × symbols).

Optionally, you may show off your fluency with exponents by writing the prime factors as compactly as possible using exponents.

The ordered list of prime factors is called the prime factorization.

For example, the prime factorization of 66 is 2 × 2 × 2 × 3.

36. Find the prime factorization of 48.

Looking at the factor trees for 48 we see the prime factorization is 2 × 2 × 2 × 2 × 3 = 48.

This can also be written as 24 × 3 = 48.

37. Find the prime factorization of each number.

(a) 24 = 2 × 2 × 2 × 3

(b) 18 = 2 × 3 × 3

(c) 51 = 3 × 17

(d) 49 = 7 × 7

(e) 80 = 2 × 2 × 2 × 2 × 5

(f) 280 = 2 × 2 × 2 × 5 × 7

videos

Video example problems have three images to click on. You can see a video step-by-step answer, a written step-by-step answer, or only the answer. If you find yet more helpful videos, please let your instructor know so that this website can be updated and improved!

Kahn Academy

Finding Factors

Divisibility Tests for 2, 3, 4, 5, 6, 9, 10

Finding Factors of a Number

YouTube Problems

play video show steps show answer Identify the products and factors: 30 = 2 × 3 × 5 The answer to a multiplication problem is the product.
The amounts being multiplied are the factors.
So the product is 30, and the factors are 2, 3, and 5.
The product is 30. The factors are 2, 3, and 5.

play video show steps show answer Identify the products and factors: 9 × 8 = 72 The answer to a multiplication problem is the product.
The amounts being multiplied are the factors.
So the product is 72, and the factors are 8 and 9.
The product is 72. The factors are 8 and 9.

play video show steps show answer Determine whether 784 is divisible by 9. The sum of the digits is 7 + 8 + 4 = 19.
Does 9 go into 19? No.
So 9 does not go into our original number either.
No.

play video show steps show answer Determine whether 5,552 is divisible by 5. The one's place value digit is 5
Is this 0 or 5? Yes.
So 5 goes into our number.
Yes.

play video show steps show answer Determine whether 2,322 is divisible by 6. The one's place value digit is 2
Is this 0, 2, 4, 6, or 8? Yes.
So 2 goes into our number.
The sum of the digits is 2 + 3 + 2 + 2 = 9.
Does 3 go into 9? Yes.
So 3 goes into our original number also.
Both 2 and 3 work, so 6 also works.
Yes.

play video show steps show answer Find all the factors of 300. Start counting, and writing the matching factor.
1 × 300. 2 × 150. 3 × 100. 4 × 75. 5 × 60. 6 × 50. 7 doesn't work. 8 doesn't work. 9 doesn't work. 10 × 30. 11 doesn't work. 12 × 25. 13 doesn't work. 14 doesn't work. 15 × 20. 16 doesn't work. 17 doesn't work. 18 doesn't work. 19 doesn't work.
Now we are up to 20, a number that already appeared. We can stop.
1, 2, 3, 4, 5, 6, 10, 12, 15, 20, 25, 30, 50, 60, 75, 100, 150, 300

play video show steps show answer Find the prime factorization of 18. Make a factor tree. However you do that, the "leaves" will be 2, 3, and 3.
So you can write 2 × 3 × 3 or you can write 2 × 32
You can write 2 × 3 × 3, or you can write 2 × 32

play video show steps show answer Find the prime factorization of 60. Make a factor tree. However you do that, the "leaves" will be 2, 2, 3, and 5.
So you can write 2 × 2 × 3 × 5 or you can write 22 × 3 × 5
You can write 2 × 2 × 3 × 5, or you can write 22 × 3 × 5

Textbook cover

Textbook Exercises for Divisibility and Factors

This list of recommended odd-numbered textbook problems is designed for a hypothetical student with a "typical" math background. If your math foundation is weak, do even more odd-numbered problems. If your math foundation is strong, do fewer.

Please read the advice on doing homework in the study skills page.

Section 2.3 (page 117) # 9, 11, 13, 15

Desired Denominators §2.1, §2.3, §3.1

How can we change a fraction so it has a desired denominator?

That is an excellent question. We will answer it soon. But first it is time for cake!

Let us consider the fraction one-third drawn as a slice of cake.

cake cutting example

Un-Reducing

The most intuitive thing to do with those big slices of cake is to keep cutting the cake.

38. Change the three-piece cake by making one cut so the cake will have six equal pieces. How many are shaded?

If we make a horizontal cut through the center, the two left-most pieces will be shaded.

We can represent this cake-cutting with numbers. We take 13 and multiply the numerator and denominator by two.

39. Change the three-piece cake by making two cuts so the cake will have nine equal pieces. How many are shaded?

If we make two horizontal cuts, the three left-most pieces will be shaded.

We can again represent this cake-cutting with numbers. We take 13 and multiply the numerator and denominator by three.

Definition

Un-Reducing a fraction represents with numbers the act of slicing the cake more. The numerator and denominator are multiplied by the same value.

Yikes. I am glad our first two definitions were admirable. Because the third was slightly sloppy. And this one talks about cake? What kind of word is "un-reducing" anyway? I don't think that is actual math jargon. Sigh. I thought we had standards.

Anyway, notice that there are four ways to write "un-reducing" one-third into two-sixths. Three are appropriate. One is wrong.

Writing Un-reducing

"Un-reduce" the fraction 13 into an equivalent fraction with a denominator of six.

(a) write two little, floating × 2, one each for numerator and denominator

(b) write a single, centered × 2 intended to work on both numerator and denominator

(c) write another fraction: × 22

(d) cross out the one and three and replace them with the larger values

Do not use (b)! This is how we write multiplication, not "un-reducing" numerator and denominator.

A moment ago we asked, "How can we change a fraction so it has a desired denominator?" Now we have one answer to that question.

If the current denominator is a factor of our desired denominator, we can multiply both numerator and denominator by something.

Enough about "un-reducing"! I don't think it is a real word anyway. I have never heard it before, Google doesn't know about it, and it keeps appearing inside quotation marks.

Reducing

Can we use a cake diagram to think about dividing both numerator and denominator of a fraction.

Sure. We could make big slices of cake by gluing smaller slices together. Maybe we use very sticky icing?

Here is a whole cake.

cake cutting example

We will prepare the whole cake by making two vertical slices and one horizontal slice to show sixths. Shade in the left-most two of the sixths.

(To be extra clear, we will use one color pen for the vertical slices, and another color for the horizontal slice.)

40. Glue cake slices together to change two-sixths into one-third.

We can represent this cake-gluing with numbers. We take 26 and divide the numerator and denominator by two.

Next we erase, and prepare the whole cake by making two vertical slices and two horizontal slice to show ninths. Shade in the left-most three of the ninths.

(To be extra clear, we will use one color pen for the vertical slices, and another color for the horizontal slices.)

41. Glue cake slices together to change three-ninths into one-third.

We can represent this cake-gluing with numbers. We take 39 and divide the numerator and denominator by three.

Here is our definition.

Definition

We reduce a fraction by dividing the numerator and denominator by the same value. That value must be a factor of both the numerator and denominator.

Since reducing is not make-believe jargon, we actually need a second definition.

Definition

A fraction is fully reduced if its numerator and denominator have no factor in common (except 1).

Many books and websites call a fully reduced fraction "simplified". We avoid that word because it has a bad tendency to be used in too many situations without careful definitions. It ends up fuzzy and vague, for many students meaning no more than "make the instructor happy".

Again there are four ways to write reducing two-sixths into one-third. Three are appropriate. One is wrong.

Writing Reducing

Reduce the fraction 26 into an equivalent fraction with a denominator of three.

(a) write two little, floating ÷ 2, one each for numerator and denominator

(b) write a single, centered ÷ 2 intended to work on both numerator and denominator

(c) write another fraction: ÷ 22

(d) cross out the two and three and replace them with the smaller values

Do not use (b)! This is how we write division, not reducing numerator and denominator.

Notice that reducing is a process. Canceling is one possible step in the process. (Reducing and canceling are related, but not the same thing!)

A moment ago we asked, "How can we change a fraction so it has a desired denominator?" Now we have another answer to that question.

If the desired denominator is a factor of our current denominator, we can try to divide both numerator and denominator by something.

Notice the word try. We cannot always reduce. If we wanted thirds, but currently have 56, we are stuck. Our desired denominator of 3 is indeed a factor of our current denominator of 6. But trying to reduce does not work with that numerator. We do not always get what we want.

An important comment is that efficiency is a convenience, not a virtue when reducing fractions.

42. Use three steps (each of dividing the numerator and denominator by 2) to fully reduce the fraction 824

That worked great!

43. Use one step (of dividing the numerator and denominator by 8) to fully reduce the fraction 824

That also worked great!

Some students like to reduce with many steps. It can feel satisfying and safe to chip away at the numbers a little bit at a time.

Other students like to reduce with one step. It can feel satisfying and powerful to find which factor is most efficient.

You can use either method.

To help us talk about the difference, we need one more definition.

Definition

The greatest common factor of two numbers is the...um...biggest factor they both have...

XKCD comic 703

Oops. That's not a definition. That's a tautology. Sorry.

Anyway, congratulations again! You are almost at the end of the second subtopic (Fraction Format) for our first big topic (Shapeshifting).

You have now thought more carefully than most people about division, factors, and reducing and "un-reducting" fractions. You know tricks for finding all the factors, and have seen that those tricks are useful when reducing a fraction.

We have not yet tried finding a common denominator for two or more fractions. When that task arrives you are prepared with a solid foundation.

One more example problem as a finale.

44. Fully reduce the fraction 330462

videos

Video example problems have three images to click on. You can see a video step-by-step answer, a written step-by-step answer, or only the answer. If you find yet more helpful videos, please let your instructor know so that this website can be updated and improved!

Kahn Academy

Equivalent Fractions

Visualizing Equivalent Fractions

Equivalent Fraction Models

YouTube Problems

(we need some problems here)

Textbook cover

Textbook Exercises for Desired Denominators

This list of recommended odd-numbered textbook problems is designed for a hypothetical student with a "typical" math background. If your math foundation is weak, do even more odd-numbered problems. If your math foundation is strong, do fewer.

Please read the advice on doing homework in the study skills page.

Section 2.1 (page 99) # 31, 33, 35, 37, 47, 49, 51, 53, 65

Section 2.3 (page 117) # 17, 19, 23, 37, 39, 41

Section 3.1 (page 151) # 17, 19, 23, 37, 45, 47, 19, 51

Fractions and Decimals §4.5

Decimals to Fractions

To change a decimal into a fraction, we just say its formal name and then reduce if possible. This is one reason why we need to remember place value names!

45. Write in fraction format.

(a) 8 hundredths = 8100 = 225

(a) 125 thousandths = 1251,000 = 18

One way to review converting decimals to fractions is to play The Decimals to Fractions Game.

Game Rules: The Decimals to Fractions Game

A moderator provides five numbers.

Students, in teams with one representative at the board, race to change three of the numbers at a time nto a correct decimal-to-fraction equation. The three numbers used become the decimal, numerator, and denominator.

How many solutions can the class find?

Example of the Decimals to Fractions Game

The five numbers are 1, 2, 5, 6, and 30.

One possible solution uses the three numbers 1, 2, and 5 to create the equation 0.5 = 12

A second possible solution uses the three numbers 2, 6, and 30 to create the equation 0.2 = 630

factor game

The five numbers are 1, 4, 8, 25, and 125.

The five numbers are 4, 8, 16, 20, and 25.

The five numbers are 3, 6, 15, 20, and 75.

Fractions to Decimals

To change a fraction into a decimal, we just treat the fraction as a division problem. We do numerator ÷ denominator = decimal.

46. Write in decimal format.

(a) 1 ÷ 5 = 0.2

(b) 3 ÷ 8 = 0.375

If we are not allowed to use a calculator, we can sometimes avoid long division. There is a clever way to change a fraction to a decimal by making the denominator a power of ten

Cleverness

125 = 125 × 44 = 4100 = 4 hundredths = 0.04

25 is a factor of 100. So we can "un-reduce" to make the denominator 100, then say the fraction's formal name.

However, this clever trick is not really a shortcut. In most real life situations, checking whether fraction denominator were factor of 10, 100, etc. slows you down a lot compared to simply using a calculator.

Repeating Decimals to Fractions

We cheated above by pretending that all decimals have a formal name.

But some decimals do not!

Definition

Repeating decimals have a never-ending pattern. We write them using a bar over the repeating portion instead of ellipsis to avoid ambiguity.

Why We Need a Bar

Which do we mean by 0.321...

0.3211111...?       0.321212121...       0.321321321...

Note that a repeating decimal has no formal name. So we cannot use the usual method to change it into a fraction.

For the three most common repeating decimals it is best to simply memorize their fraction equivalents.

Memorize These

One-third = ?

Two-thirds = ?

One-sixth = ?

There is a method for changing a repeating decimal into a fraction, but it is tricky.

Knowing how to do this is not required. We have not discussed equations yet. This will not appear on any homework assignment, quiz or test!

The method involves setting the repeating decimal equal to y, multiplying both sides of the equation by a 10 one or more times, and then subtracting the two equations before solving.

It makes more sense when you see it happen.

47. Change 0.222222... into a fraction.

Set the repeating decimal equal to y.
    y = 0.222222...
Multiply both sides by 10.
    10y = 2.22222...
Subtract the first from the second.
    9y = 2
Divide both sides by 9 to get y by itself.
    y = 29

videos

Video example problems have three images to click on. You can see a video step-by-step answer, a written step-by-step answer, or only the answer. If you find yet more helpful videos, please let your instructor know so that this website can be updated and improved!

YouTube Problems

(we need some problems here)

Textbook cover

Textbook Exercises for Fractions and Decimals

This list of recommended odd-numbered textbook problems is designed for a hypothetical student with a "typical" math background. If your math foundation is weak, do even more odd-numbered problems. If your math foundation is strong, do fewer.

Please read the advice on doing homework in the study skills page.

Section 4.5 (page 238) # 5, 9, 11, 21, 23, 25, 27, 29, 31

Ratios and Rates §5.1, §5.2

We are not done yet with thinking about fractions.

So far all of our fractions were proper fractions. They were cake slice numbers. The denominator counted the number of pieces. The numerator counted the shaded pieces.

Cooks or carpenters deal with those fractions daily. But for many people their real-life fractions are not like that. Their fractions do have a sense of "total pieces" and "shaded pieces".

When is a fraction not a fraction? Cue the suspenseful music...

Let's notice something interesting about fractions and division.

We know that we can change fractions to decimals by doing division. In this sense fractions are like division problems waiting to happen.

A Fraction as Division Waiting to Happen

15 = 1 ÷ 5 = 0.2

We often reduce fractions. In this sense fractions are not like division. When we divide one number by another we do not reduce the numbers first.

This Hurts My Brain

105 = 10 ÷ 5 = 2 ÷ 1 = 2

It looks really weird to "reduce" 10 ÷ 5 and change it into 2 ÷ 1 in the middle of a division problem. No one does that!

We talk about "equivalent fractions" but never "equivalent division problems".

Snoopy vulture So our first clue is that fraction format is a good way to patiently lurk like a vulture, spying on a situation that does not yet involve division but might soon.

This happens frequently when we compare numbers.

A package of 24 energy drinks costs $35.99. Does anyone care how much one drink costs? Maybe. We lurk, waiting to see if division will happen.

A contractor submits a $2,800 bid for installing a 120 foot long fence. Does anyone care about how much this costs per foot? Maybe. We lurk, waiting to see if division will happen.

So we found a reason to deal with a pair of numbers as if they were the numerator and denominator in a fraction, even though the numbers have meaning very unlike "total pieces" and "shaded pieces".

This is the job for a ratio.

Definition

A ratio is a comparison of two numbers, usually written as a fraction.

Some ratios do have a sense of "total pieces" and "shaded pieces".

48. Snoopy interviewed 10 dentists and 8 of them approved of a certain breath mint. Write this comparison as a ratio.

Other ratios do not have a sense of "total pieces" and "shaded pieces".

49. The Red Baron can fly 38 miles in 40 minutes. Write this comparison as a ratio.

There are even ratios with a hidden sense of "total pieces" and "shaded pieces".

50. A classroom has 4 left-handed people and 16 right-handed people. Write this comparison as a ratio.

We could look at the ratio and find the whole (20 total people). But the provided numbers do not explicitly tell us the whole.

Snoopy upside down Notice that, like Snoopy pretending to be the Flying Ace, we can even write a ratio upside down. This does not lose any meaning. It is not confusing.

How different from a normal fraction, in which the numerator and denominator count "total pieces" and "shaded pieces". Those would have their meaning changed if written upside down.

51. A classroom has 4 left-handed people and 16 right-handed people. Write this comparison as a different ratio, "upside down" compared to your previous answer.

Often it does not make sense to reduce a ratio. If I reduced Snoopy's breath mint ratio I would switch from an 810 which involved numbers he actually counted to 45. Neither the 4 nor 5 represent something from the real world. He did not interview 5 dentists, of whom 4 approved. Yes, the fractions have the same value. But it would be silly to reduce this ratio for no reason.

breath mints for dogs Maybe his friends were also asking dentists about other brands of breath mints. But his friends were lazier, and each only spoke to five dentists. In that situation reducing his ratio might be useful, so it could be more easily compared to the ratios representing his friends' data.

Reducing ratios can be sensible. The point is merely that it is not always sensible, unlike how most math classes have a rule that fraction answers should be fully reduced.

Similarly I would lie if I changed Snoopy's ratio to 80100 this makes it sound like he did a lot more work by interviewing 100 dentists. (Note that many people, especially in advertising, do change ratios in this manner.)

Tangentially, not all ratios are written as fractions. The ratio 810 can also be written as 8 to 10 or as 8 : 10. But in real life, almost no one but the writers of SAT tests and gamblers write ratios that way.

Some ratios include words to label the numbers.

A Ratio with Word Labels

A child has 18 green candies and 9 yellow candies.

We could write this rate as 18 greens9 yellows.

Time for another definition.

Definition

A rate is a kind of ratio in which the two numbers have different labels.

Once again, we could change a rate into an equivalent fraction but typically do not do so to preserve the record of a real life situation.

When we stop lurking and finally do the division problem, our answer should retain those labels.

Division changes a rate into a different rate whose second value is 1.

This happens so often there is a special name for it.

Definition

A unit rate is a rate whose second value is 1.

52. A child has 18 green candies and 9 yellow candies. Write this as a unit rate.

The unit rate is 2 greens1 yellow

This is subtle. 18 ÷ 9 = 2. But our answer is not the plain number 2.

Because we keep both labels, and we write the comparison as a fraction, we still have a rate.

53. Kim solved a 300 piece jigsaw puzzle in 90 minutes. Write this as a unit rate.

The unit rate is about 33 pieces1 minute

We often express unit rates in English by omitting the denominator of 1 and using the word "per" to represent the fraction bar between the unit labels.

54. A child has 18 green candies and 9 yellow candies. What is the average number of greens per yellow?

There are 2 greens per yellow.

55. Kim solved a 300 piece jigsaw puzzle in 90 minutes. What was her average rate of pieces solved per minute?

Kim solved about 33 pieces per minute.

Any rate can be changed into a unit rate by treating the rate as a division problem. Doing "top ÷ bottom" simplifies the rate into a unit rate.

Unit rates make comparisons easy. We might prefer either the biggest or the smallest unit rate.

Shopping Example

I am shopping for fancy hand lotion. Which is a better deal, 10 ounces for $5 or 24 ounces for $8?

If we find ounces per dollar we want the most ounces per dollar.

In the first case, imagine someone with $1 shopping at a gas station. They want the most gas for their $1.

If we find dollars per ounce we want the least cost per ounce.

In the second case, imagine someone with who needs 1 ounce of something shopping at the bulk food section of a grocery store. They need an unusual ingredient for a recipe, and want to spend as little as possible for the correct amount.

Congratulations again! You are at the end of the second subtopic (Fraction Format) for our first big topic (Shapeshifting).

You have now thought more carefully than most people about writing one number on top of another, and waiting before doing division. That might not sound like a significant accomplishment, but it will help you in future math topics.

videos

Video example problems have three images to click on. You can see a video step-by-step answer, a written step-by-step answer, or only the answer. If you find yet more helpful videos, please let your instructor know so that this website can be updated and improved!

Kahn Academy

Intro to Ratios

Ratio Review

Basic Ratios

Part to Whole Ratios

Intro to Rates

Solving Unit Rate Problem

Solving Unit Price Problem

Mathispower4u

Rates and Unit Rates

YouTube Problems

play video show steps show answer Write the ratio "85 to 97" in fraction notation. Simply write one number above the other, the fraction bar represents the word "to"
8597
8597

play video show steps show answer Write the ratio "0.34 to 124" in fraction notation. Simply write one number above the other, the fraction bar represents the word "to"
0.34124
0.34124

play video show steps show answer A twelve pound shankless ham contains sixteen servings. What is the rate in servings per pound? The word "per" means division. So we do servings ÷ pounds = 16 ÷ 12 ≈ 1.3 servings per pound 1.3 servings per pound

play video show steps show answer A car will travel 464 miles on 14.5 gallons of gasoline. What is the rate in miles per gallon? The word "per" means division. So we do miles ÷ gallon = 464 ÷ 14.5 = 32 miles per gallon 32 miles per gallon

play video show steps show answer A sixteen ounce bag of salad greens costs $2.39. Find the unit price in cents per ounce.EA sixteen ounce bag of salad greens costs $2.39. Find the unit price in cents per ounce. The word "per" means division. So we do cents ÷ ounce = 239 ÷ 16 ≈ 15 cents per ounce 15 cents per ounce

Textbook cover

Textbook Exercises for Ratios and Rates

This list of recommended odd-numbered textbook problems is designed for a hypothetical student with a "typical" math background. If your math foundation is weak, do even more odd-numbered problems. If your math foundation is strong, do fewer.

Please read the advice on doing homework in the study skills page.

Section 5.1 (page 264) # 1, 3, 5, 11, 13, 15, 17, 21, 23, 29

Section 5.2 (page 272) # 1, 3, 5, 7, 11, 13, 17, 19, 21, 29

Ten Exercises

Try these ten exercises on scratch paper. Work in a study group if you can! Notice where your notes need improvement. After you are very happy with your answers, you can use this form to ask me to check your work. Can you get at least 8 out of 10 correct?

1. Consider the following six numbers: 315, 1101, 7628, 13025, 110124, and 876. Which are divisible by 3?

2. Consider the following six numbers: 315, 1101, 7628, 13025, 110124, and 876. Which are divisible by 4?

3. How many factors does 126 have?

4. What is the prime factorization of 126?

5. Fully reduce the fraction 16120

6. Change 0.45 to a fraction. Remember to simplify.

7. What is the decimal notation for four-twentieths?

8. The Leaning Tower of Pisa is 184.5 feet tall and leans 13 feet from its base. What is the ratio of the distance it leans to its height?.

9. The area is 0.75 square miles. The population is 35,427 people. What is the unit rate of people per square mile?

10. Brand M soup weighs 54 ounces and costs $4.79. Brand T soup weighs 59 ounces and costs $5.99. Which is the better buy?

Random Exercises

Try these exercises on scratch paper. Work in a study group if you can! Notice where your notes need improvement. Check your work when you are done.

Thinking About Division

Divisibility and Factors

Desired Denominators

Fractions and Decimals

Ratios and Rates


Percent Format

Decimal Point Scoots §4.3

As we transition from fraction format to percent format, let us review what happens when we multiply or divide by powers of ten.

56. Multiply by powers of ten. Use a calculator if you wish.

(a) 1,234.56

(b) 12,345.6

(c) 123,456

The decimal point scoots to the right.

How many scoots just happened compared to the zeroes in the power of ten?

Now let's look at division.

57. Divide by powers of ten. Use a calculator if you wish.

(a) 12.3456

(b) 1.23456

(c) 0.123456

The decimal point scoots to the left.

How many scoots happen compared to the zeroes in the power of ten?

The next step is to multiply and divide by decimal amounts. These are powers of one-tenth.

58. Multiply by powers of ten. Use a calculator if you wish.

(a) 12.3456

(b) 1.23456

(c) 0.123456

The decimal point scoots to the left.

How many scoots just happened compared to the zeroes in the power of one-tenth?

So dividing by powers of ten works just like multiplying by powers of one-tenth.

59. Divide by powers of ten. Use a calculator if you wish.

(a) 1,234.56

(b) 12,345.6

(c) 123,456

The decimal point scoots to the right.

How many scoots just happened compared to the zeroes in the power of one-tenth?

So multiplying by powers of ten works just like dividing by powers of one-tenth.

60. Your turn, to practice multiplication for reliability and speed. Do the following without a calculator.

(a) 31,467

(b) 314,670

(c) 31.467

(d) 3,146.7

(e) 3.1467

(f) 0.031467

(g) 3,146,700

(h) 0.31467

(i) 314.67

61. Your turn again, to practice division for reliability and speed. Do the following without a calculator.

(a) 5.9028

(b) 590.28

(c) 0.059028

(d) 5,902.8

(e) 59,028

(f) 0.59028

(g) 0.00059028

(h) 5,902,800

(i) 59.028

videos

Video example problems have three images to click on. You can see a video step-by-step answer, a written step-by-step answer, or only the answer. If you find yet more helpful videos, please let your instructor know so that this website can be updated and improved!

YouTube Problems

(we need some problems here)

Textbook cover

Textbook Exercises for Decimal Point Scoots

This list of recommended odd-numbered textbook problems is designed for a hypothetical student with a "typical" math background. If your math foundation is weak, do even more odd-numbered problems. If your math foundation is strong, do fewer.

Please read the advice on doing homework in the study skills page.

Section 4.3 (page 216) # 23, 25, 27

Four Replacements for % §6.1

Our society is used to using numerical scales to rate how nice things are.

Here is table to complete. We'll just make up answers, rating things.

Rating Scales

Scale of
1 to 5
Scale of
1 to 10
Scale of
1 to 100
Jogging   
Banana Runts   
Elevator Music   

four stars In English, the popularity of product reviews and Auto Club travel guides means a "star" is assumed to be a rating on a scale of 1 to 5.

There is no nice name in English for how something rates on a scale of 1 to 10.

percent symbol

The name for how something rates on a scale of 1 to 100 used to be called per cent, meaning "per 100" since the word "cent" means "100". But over time those words became customarily squished together, and now we say percent.

This idea of "per 100" or "out of 100" is so important that we should be more formal.

Definition

Percent means "out of 100".

Therefore the symbol % can be replaced with "out of 100".

By the way, we know four ways to do "out of 100" with arithmetic.

Let's do each of those four operations from the definition of percent.

62. Use each of the four ways to do "out of 100" with arithmetic.

These examples will look incomplete. They are incomplete. Very soon we will see when each is actually useful. For now we are merely warming up.

If you are a visual learner, it might help to picture percentages using a grid of 100 boxes or a circle with 100 tic marks. These can help us draw pictures for "out of 100". The diagrams are especially helpful for thinking carefully about small percentages.

63. Use the grid to represent percentages.

square with 100 boxes

64. Use the circle to represent percentages.

circle with 100 tic marks

In real life percents often come as a set of values that add up to 100%, and a missing value must be found using subtraction.

65. How much orange juice is sold?

percent juices pie chart

But that type of problem only involves subtraction, so it is too boring to appear again in this class.

videos

Video example problems have three images to click on. You can see a video step-by-step answer, a written step-by-step answer, or only the answer. If you find yet more helpful videos, please let your instructor know so that this website can be updated and improved!

YouTube Problems

play video show steps show answer Write 14.7% as a decimal. Use RIP LOP to remind us to go "left out of percent" two scoots, to get 0.147 0.147

play video show steps show answer Write 0.38 as a percent. Use RIP LOP to remind us to go "right into percent" two scoots, to get 38% 38%

play video show steps show answer Write 65% as a fraction. Replace the % symbol with "write the number over 100", to get 65100 = 1320 1320

play video show steps show answer Write 118 as a percent. First do "top ÷ bottom" to change 118 into 1.375
Then use RIP LOP to remind us to go "right into percent" two scoots, to get 137.5%
137.5%

Textbook cover

Textbook Exercises for Four Replacements for %

This list of recommended odd-numbered textbook problems is designed for a hypothetical student with a "typical" math background. If your math foundation is weak, do even more odd-numbered problems. If your math foundation is strong, do fewer.

Please read the advice on doing homework in the study skills page.

Section 6.1 (Page 316) # 1, 3, 5

Percents, Decimals, and Fractions §6.1

Remember our four ways to create "out of 100" using arithmetic?

Let's explore which help us change the format of numbers. We can change among decimal format, percent format, and fractions.

Percent to Decimal

What if we start in percent format and want to switch to decimal format?

We need to replace the % symbol using arithmetic. Which of the four versions of what percent means is the most helpful in this situation?

66. Write 47.1% as a decimal.

The easiest replacement is "two decimal point scoots to the left" to get 0.471

67. Your turn again, to practice for reliability and speed. Do the following without a calculator.

(a) 0.03

(b) 0.3

(c) 1.03

(d) 0.103

Decimal to Percent

What if we start in decimal format and want to switch to percent format?

Now we need to create the % symbol by "un-doing" one of those four options. Which of the four versions of what percent means is the most helpful in this situation?

68. Write 0.75 as a percent.

If we stick a % symbol on the number we also need to "undo" it to be fair. After all, we cannot change the number for no reason! The easiest un-doing is "two decimal point scoots to the right" to get 75%.

69. Your turn again, to practice for reliability and speed. Do the following without a calculator.

Let's do more examples of changing from decimal format to percent format.

(a) 4%

(b) 230%

(c) 37.5%

(d) 66.6...%

RIP LOP

Definition

The acronym RIP LOP summarizes moving between decimal format and percent format. It stands for Right Into Percent, Left Out of Percent.

Notice that RIP LOP does not tell us "two places of decimal scoots". But we can remember that we always scoot twice.

Percents are always about 100, and 100 always has two zeroes, and we saw earlier that multiplying or dividing by 100 always causes two scoots.

70. Write 0.76 as a percent.

71. Write 3.3% as a decimal.

Percent to Fraction

What if we start in percent format and want to switch to fraction format?

We again need to replace the % symbol using arithmetic. Which of the four versions of what percent means is the most helpful in this situation?

72. Write 42% as a fraction.

The easiest replacement is "writing the number as a fraction with denominator 100" to get 42100 which reduces to 2150

Let's do another example.

73. Write 80% as a fraction.

80% = 80100 = 810 = 45

In the previous two example we dealt with a whole number in percent format. What about changing a fraction in percent format into a fraction not in percent format?

A different one the four versions of what percent means is the most helpful in this situation!

74. Write 2 35 % as a fraction.

This time we will replace the % symbol with × 1100

2 35 % = 135 % = 135 × 1100 = 13500

We have not talked about fraction multiplication yet. If this example does not make sense to you, please file it away in the back of your mind until we do discuss fraction multiplication.

Fraction to Percent

What if we start in fraction format and want to switch to percent format?

A few fractions are too easy. We can simply look at 51100 and see that it is "51 out of 100" or 51%.

In general it is quickest to use division to change the fraction into decimal format, and then use RIP LOP.

75. Write 14 as a percent.

14 = 1 ÷ 4 = 0.25 = 25%

76. Write 78 as a percent.

78 = 7 ÷ 8 = 0.875 = 87.5%

Notice that our percent format answer may include a repeating decimal.

77. Write 19 as a percent.

19 = 1 ÷ 9 = 0.111... = 11.111...%

Congratulations! You are at the end of the third subtopic (Percent Format) for our first big topic (Shapeshifting).

You have now thought more carefully than most people about the relationship between decimals, percentages, and fractions. You word problems arrive you are prepared with a solid foundation.

videos

Video example problems have three images to click on. You can see a video step-by-step answer, a written step-by-step answer, or only the answer. If you find yet more helpful videos, please let your instructor know so that this website can be updated and improved!

Kahn Academy

Multiplying a Decimal by a Power of 10

Dividing a Decimal by a Power of 10

Dividing a Decimal by a Power of 10: Pattern

Multiplying Decimals by 10, 100, and 1000 (worksheet)

Dividing Decimals by 10, 100, and 1000 (worksheet)

morgankenneth12

Percents, Fractions, Decimals Review

Mathispower4u

Introduction to Percent

Relating Fractions, Decimals, and Percents - Example 1

Relating Fractions, Decimals, and Percents - Example 2

YouTube Problems

(we need some problems here)

Textbook cover

Textbook Exercises for Percents, Decimals, and Fractions

This list of recommended odd-numbered textbook problems is designed for a hypothetical student with a "typical" math background. If your math foundation is weak, do even more odd-numbered problems. If your math foundation is strong, do fewer.

Please read the advice on doing homework in the study skills page.

Section 6.1 (Page 316) # 11, 13, 15, 17, 25, 31, 33, 35, 37, 41, 43, 45, 49, 59, 61, 63, 65

Ten Exercises

Try these ten exercises on scratch paper. Work in a study group if you can! Notice where your notes need improvement. After you are very happy with your answers, you can use this form to ask me to check your work. Can you get at least 8 out of 10 correct?

1. An easy way to divide a number by 1,000 is to scoot the decimal point ___ places to the _____.

2. An easy way to multiply a number by 0.01 is to scoot the decimal point ___ places to the _____.

3. Write 5.5% as a decimal.

4. Write 0.8911 as a percent.

5.Write 0.8% as a fraction.

6. Write 25 38 % as a fraction.

7. Write the fraction 58 as a percent.

8. Write the fraction 1750 as a percent.

9. After we change two-thirds into percent format, do its digits repeat endlessly?

10. Use what you have memorized to write 33 13 percent as a fraction.

Random Exercises

Try these exercises on scratch paper. Work in a study group if you can! Notice where your notes need improvement. Check your work when you are done.

Decimal Point Scoots

Four Replacements for %

Percents, Decimals, and Fractions


Measurement Units

We begin our topic of measurement unit conversions with a definition.

Definition

A measurement's unit is the word that labels the measurement.

Unit = Word Label for Measurements

I am 63 inches tall. The measurement unit is inches.

By convention we use plurals of such words when talking about units.

Plural, Please

A large box is 1 yard tall. The measurement unit is yards.

Note that unit rates have two units.

A Rate Comparing Two Units

A speed limit is 30 miles per hour. The measurement units are miles and hours.

There are measurements that use more than two units, but we will not use them in this class. As one example, thinking carefully about a 20-watt light bulb requires considering six units!

Watt?

A watt is the rate of power of moving a weight of one kilogram at a speed of one meter per second when resisted by a deceleration of one meter per second per second.

Units and Their History (none)

Time to take a short break from mathematics to talk about this history of measurement units. (A thank you to Wikipedia for providing much of the following history lesson.)

Where did we get our measurement units? The older system of units from Europe is now named Standard, American, or Imperial units. The newer system is named SI units, but most people still call it by its older name, the metric system.

Length

We'll start by talking about units for measuring length.

Many European countries used lengths named the "inch", "foot", and "yard".

European historical records from the twelfth century show much more care in measuring the inch than the foot or yard. Apparently a town would measure an inch by taking the average value of thumb width of a few men, which produced a result that had little regional variation. Carpentry and other crafts needed a standardized inch more than a foot or yard.

Having twelve inches in one foot was a remnant of the Roman Empire. So a foot was always twelve inches. This was much longer than nearly everyone's feet, but could be estimated by stepping heel-toe heel-toe in boots.

For many centuries a "yard" meant many things in Europe. The common theme was that a yard was about a stride length. In different places a yard was either three feet, average waist circumference, or two cubits (elbow to elbow when touching fingertips). In places that based yards on feet, a yard was always three feet.

The original mile was five thousand Roman "paces". People stopped using these "paces" but kept the mile. The result is that one mile is 5,280 feet. The details involve furlongs, and you can read more here.

The metric system was created in France, beginning in 1791. In 1793 the French Academy of Sciences adopted as the country's official unit of length a meter, defined as one ten-millionth of the distance from the Equator to the North Pole through Paris.

How long was this meter? It is the same meter we use today. The expedition sent to measure this distance was only off by by one-fiftieth of a percent. This was remarkable accuracy considering the technology available then! But it does mean the meter we use is not quite the originally intended length, so people felt free to adjust the definition in later years.

In 1889 the first General Conference on Weights and Measures established a new definition. This group built an International Prototype Metre: a bar composed of an alloy of ninety percent platinum and ten percent iridium, marked with two lines whose distance apart was measured at the melting point of ice. The purpose was to keep the same length but have a physical example that could be easily copied by anyone visiting Paris.

Many of the rulers we use have one side with inches and feet, and the other side with centimeters and millimeters. Can you look at such a ruler and see that one inch is about two-and-a-half centimeters? Interest in redefining measurement units for length resurfaced in the late 1950s. Since the meter had a proptype but the inch did not, in 1958 the inch was redefined as exactly 2.54 centimeters.

In 1960 the meter was redefined again, this time as 1,650,763.73 wavelengths of the orange-red emission line in the electromagnetic spectrum of the krypton-86 atom in a vacuum. This definition again aimed to keep the length unchanged while creating a physical example that could be duplicated almost exactly by anyone, anywhere in the world, with common laboratory equipment.

In 1983 the meter was redefined for the last time. Now it is the distance traveled by light in free space in 1299,792,458 of a second. This final definition again kept the length unchanged, and finally created a physical example that could be duplicated exactly by anyone, anywhere in the world, with common laboratory equipment. Much easier and less expensive than visiting Paris to measure a metal bar!

A meter is broken up into 100 centimeters or 1,000 millimeters. (To help remember this think about 100 years in one century and 1,000 years in one millenium.)

Mass

The Roman Empire used weights from which we get the words "pounds" and "ounces". After the Roman Empire, an ounce saw various definitions across Europe, usually 450 or 480 grains of barleycorn because that grain is very uniform. The Roman pound was 12 ounces. But soon after the Roman Empire ended, most European merchants switched to a sixteen ounce pound.

For the metric system, a gram was defined as the mass of one cubic centimeter of water. This is a small amount! It is useful in the laboratory, but not as practical for commerce. So 1,000 grams (a kilogram) is used more in normal daily life.

(A question to ponder: Why wasn't one cubic meter of water used as the standard mass? That would be a logical choice, since a meter is the standard length.)

As an aside, scientists use the word mass to denote how much physical stuff an object is made up of. This is different than weight because it is independent of an object's location. The weight of an object changes proportionally to the local gravity.

78. The moon's gravity is one-sixth as strong as the Earth's gravity. If I weigh 162 pounds on Earth, how much do I weigh on the moon?

162 pounds ÷ 6 = 27 pounds

(My mother used to have a t-shirt that said, "I'm not overweight, I'm just on the wrong planet" because she had a thyroid condition that meant she would always be overweight.)

Volume

Volume was not used much until the eighteenth century. A standard size bag was much more difficult to measure, let alone construct, than a standard length rod/cord or standard weight of barleycorn grains.

For most of history, volume measurements were not used. When they were used, they were merchants' jargon for the volume taken up by a certain weight of a certain trade good. For example, a gallon was originally the volume of eight pounds of wheat.

In modern times the volumes of the American/Standard/Imperial system have been standardized.

Bakers and canners might have the advantage of already knowing these conversion rates by heart.

Is there an easy way to remember the parts of a gallon? You betcha.

Here is our gallon.

parts of a gallon

If we chop it into fourths we get quarts. The name quarts is like "quarters".

parts of a gallon

Each quart is two pints.

parts of a gallon

Each pint is two cups. Think about two short one-cup milk cartons stacked on top of each other to become the size of a one-pint carton of whipping cream.

parts of a gallon

Some students prefer to combine all of those diagrams into a single, more complicated picture.

parts of a gallon

Mind your P's and Q's

Pubs in England used to give credit to regular customers. People would only get paid once per month, and in the meanwhile the bartender would mark p's and q's (for pints and quarts) below their name on the wall behind the bar. At the end of the month this record would show what they owed to bar.

The expression "Mind your p's and q's" meant "Don't drink too much before payday!"

Yet nowadays we usually say that phrase to young children!

Be aware that there is a different unit of volume called a "dry quart" which is not quite the same amount as liquid quarts. We will not use in Math 20. But if you are part of the Willamette Valley grass seed business and shop for bushels of grain the dry quart is important! (It is almost the same amount as a liquid quart, so for grocery shopping no one notices the difference.)

Since a gram was defined as the mass of one cubic centimeter of water, that same size cube was used to define volume in the metric system. A milliliter was defined as the volume of one cubic centimeter. Note that some professions use "cubic centimeter" or cc instead of milliliter.

A milliliter is pretty small! So the liter (1,000 milliliters) is the size more appropriate for most practice uses.

(A question to ponder: Why wasn't a cubic meter used as the standard volume? That would be a logical choice, since a meter is the standard length.)

videos

Video example problems have three images to click on. You can see a video step-by-step answer, a written step-by-step answer, or only the answer. If you find yet more helpful videos, please let your instructor know so that this website can be updated and improved!

Math Antics

Intro to the Metric System

YouTube Problems

(we need some problems here)

Textbook cover

Textbook Exercises for Units and Their History

This list of recommended odd-numbered textbook problems is designed for a hypothetical student with a "typical" math background. If your math foundation is weak, do even more odd-numbered problems. If your math foundation is strong, do fewer.

Please read the advice on doing homework in the study skills page.

(Our textbook has no exercises for this topic.)

One Step Conversions §7.1, §7.3

What happens when we convert between measurement units?

These problems involve either multiplying or dividing.

Consider two examples that use the fact that there are 12 inches in 1 foot.

79. 7.5 feet is how many inches?

7.5 feet × 12 = 90 inches

In the above example we split apart each foot into twelve inches. You can imagine breaking, distributing, or shattering a bunch of one-foot rulers.

We will say a foot is multiple inches, in the same way that 15 is multiple 3s. In both cases the bigger thing is several copies of the smaller thing.

Feet are multiple inches. Yards are multiple feet or inches. Miles are multiple yards, or feet.

Pints are multiple ounces. Quarts are multuiple pints or ounces. Gallons are multiple quarts, pints, or ounces.

Centimeters are multiple millimeters. Meters are multiple centimeters or millimeters. Kilometers are multiple meters, centimeters or millimeters.

80. 30 inches is how many feet?

30 inches ÷ 12 = 2.5 feet

In the above example we grouped together a couple sets of twelve inches to each make one foot. You can imagine fusing, bundling, or gluing inches into one-foot rulers.

We will say an inch is a factor of a foot, in the same way that 3 is a factor of 15. In both cases several copies of the smaller thing make the larger thing.

Yards are a factor of miles. Feet are a factor of yards or miles. Inches are a factor of feet, yards, or miles.

Quarts are a factor of gallons. Pints are a factor of quarts or gallons. Ounces are a factor of pints, quarts or gallons

Meters are a factor of kilometers. Centimeters are a factor of meters or kilometers. Millimeters are a factor of centimeters, meters or kilometers.

So we can see the following guidelines:

Either direction happens in one step.

Definition

A one-step unit conversion can happen when we switch a measurement's unit and we know the rate that compares the old and new units.

81. Your turn, to practice one-step unit conversions for reliability and speed.

(a) We are going to inches, which are smaller than feet, so we multiply to get 69 inches

(b) We are going to feet, which are bigger than inches, so we divide to get 16.5 feet

(c) We are going to cups, which are smaller than gallons, so we multiply to get 192 cups

(d) We are going to gallons, which are bigger than cups, so we divide to get 2.5 gallons

(a) We are going to ounces, which are smaller than pounds, so we multiply to get 2,432 ounces

(b) We are going to pounds, which are bigger than ounces, so we divide to get 9.5 pounds

videos

Video example problems have three images to click on. You can see a video step-by-step answer, a written step-by-step answer, or only the answer. If you find yet more helpful videos, please let your instructor know so that this website can be updated and improved!

YouTube Problems

play video show steps show answer How many yards is 10.5 feet? There are 3 feet in 1 yard. We are bundling groups of feet into fewer yards, so we use ÷3.
10.5 ÷ 3 = 3.5 yards
3.5 yards

Textbook cover

Textbook Exercises for One Step Conversions

This list of recommended odd-numbered textbook problems is designed for a hypothetical student with a "typical" math background. If your math foundation is weak, do even more odd-numbered problems. If your math foundation is strong, do fewer.

Please read the advice on doing homework in the study skills page.

Section 7.1 (Page 371) # 1, 5, 7, 9, 11, 31

Section 7.3 (Page 388) # 1, 5, 11, 31, 39, 41

More About SI Prefixes (none)

We have already seem some examples of metric prefixes:

These examples are part of a general pattern.

Here are the metric prefixes used in these math lectures:

In this list (unit) is a placeholder for the plain unit, whether meters, grams, or liters.

These are prefixes are part of a longer list.

kilo hecto deca (unit) deci centi milli micro
× 1,000 × 100 × 10 × 1 ÷ 10 ÷ 100 ÷ 1,000 ÷ 1,000,000

Strangely, there are no prefixes for the two spots between milli and micro (for ten-thousandths and hundred-thousandths of a unit).

Even though we do not use hecto-, deca-, or deci- it is still useful to be familiar with this longer list. Why? Because of a shortcut!

But for the shortcut we do not write the whole list. Just the initials are sufficient.

SI Prefix Initials

k      h      d      unit      d      c      m      •      •      micro

Many math students memorize a cute saying (try making your own!) and remember the acronym for the first seven spots. For example, "King Henry does usually drink chocolate milk" or "Killer hobos dance under dazzling crystal mobiles."

Writing the acronym at the top of your homework and your test scratch paper is a great idea when doing SI-to-SI unit conversions. There is a shortcut!

SI One-Step Conversion Shortcut

To convert between metric prefixes, first write the list of all SI prefix initials.

Then count decimal point scoots: for every "place" you move right or left along the list of initials, move the decimal point the same way.

82. How many millimeters is 2.8 centimeters?

83. A calculator is 16 centimeters long. Change this to meters and millimeters.

84. A backpack weighs 6,480 grams. Change this to kilograms.

85. A shampoo bottle contains 450 cubic centimeters. Change this to milliliters and liters.

videos

Video example problems have three images to click on. You can see a video step-by-step answer, a written step-by-step answer, or only the answer. If you find yet more helpful videos, please let your instructor know so that this website can be updated and improved!

Mathispower4u

Metric Unit Conversion

YouTube Problems

play video show steps show answer How many meters is 6 kilometers? On the list of SI prefixes, move 3 scoots right from "kilo" to "plain units", to get 6,000 m 6,000 m

play video show steps show answer How many centimeters is 8.7 millimeters? On the list of SI prefixes, move 1 scoot left from "milli" to "centi", to get 0.87 cm 0.87 cm

play video show steps show answer Something is 0.5 cm wide. Express this width in meters and millimeters. For meters, on the list of SI prefixes, move 2 scoots left from "centi" to "plain units", to get 0.005 m
For millimeters, on the list of SI prefixes, move 1 scoot right from "centi" to "milli", to get 5 mm
0.005 m and 5 mm

play video show steps show answer Marvin L. Bittinger is 1.8542 meters tall. Express this height in centimeters and millimeters. For centimeters, on the list of SI prefixes, move 2 scoots right from "plain units" to "centi", to get 185.42 cm
For millimeters, on the list of SI prefixes, move 3 scoots right from "plain units" to "milli", to get 1,854.2 mm
185.42 cm and 1,854.2 mm

play video show steps show answer How many liters is 3,080 milliliters? On the list of SI prefixes, move 3 scoots left from "milli" to "plain units", to get 3.080 L 3.080 L

play video show steps show answer How many milliliters is 0.25 liters? On the list of SI prefixes, move 3 scoots right from "plain units" to "milli", to get 250 mL 250 mL

play video show steps show answer How many grams is 3.8 kilograms? On the list of SI prefixes, move 3 scoots right from "kilo" to "plain units", to get 3,800 g 3,800 g

play video show steps show answer How many grams is 2,200 milligrams? On the list of SI prefixes, move 3 scoots left from "milli" to "plain units", to get 2.200 g 2.200 g

play video show steps show answer How many micrograms is 0.37 milligrams? On the list of SI prefixes, move 3 scoots right from "milli" to "micro", to get 370 mcg 370 mcg

Textbook cover

Textbook Exercises for More about SI Prefixes

This list of recommended odd-numbered textbook problems is designed for a hypothetical student with a "typical" math background. If your math foundation is weak, do even more odd-numbered problems. If your math foundation is strong, do fewer.

Please read the advice on doing homework in the study skills page.

Section 7.1 (Page 371) # 15, 19, 21, 23, 27, 39

Section 7.3 (Page 388) # 15, 17, 21, 25, 29

Exponents and Square Roots §1.7, §2.3, §4.6, §7.2

Exponents

Consider this very simple square.

square foot

We know the area of a rectangle (which includes a square) is length × width. So the area of that square is 1 foot × 1 foot = 1 square foot.

The answer is labeled with square feet because it is counting how many squares it takes to cover the area.

If we changed each side from feet to inches, how should we write the area?

square inches

Now the area is 12 inches × 12 inches = 144 square inches.

The answer is labeled with square inches because it is counting how many squares it takes to cover the area, using smaller squares this time.

square inches again

Sometimes square inches are written abbreviated as sq. in. or as in2.

Similar abbreviations are used with other square units. Square feet can be sq. ft. or ft2. Square centimeters can be sq. cm. or cm2.

Later in the class, when we deal with shapes that are not rectangles, it will help to think of answers written in "square units" not as multiplication problem answers but as counting squares.

Usually we think of an exponent as its own number. It floats above its "base" and tells you how many times to multiply.

4 3 = 4 × 4 × 4 = 64

But for measurement units it can sometimes help to think of an exponent of 2 as the second half of a two-part number. It is saying "instead of a line of the base length, we are drawing a big square and then counting little squares".

(4 inches)2 = 4 inches × 4 inches = 16 square inches

86. What is the difference between these two amounts?

22 ft              2 ft2

The first measures a distance, a length covered by 4 rulers each one foot long.

The second measures an area, a flat space covered by 2 squares each one square foot.

Square Roots

We actually just talked about square roots! You might not have noticed because we did not make that clear. Let's add clarity with a definition.

Definition

The square root of a number is the amount that, when multiplied by itself, gets to that number.

For example, the square root of 25 is 5 because 5 × 5 = 25.

square root diagram

We usually write square roots with a symbol.

square root symbol

But do not forget the definition just because you are looking at a symbol!

The picture we used earlier of a square foot broken up into inches...

square inches

...shows us that the square root of 144 is 12.

87. Use a calculator to find the square root of the following numbers: 4, 6, 8, and 16.

Why are only some of those answers whole numbers?

Definition

When we multiply a whole number by itself, we call the answer a perfect square.

Perfect squares have nice square roots. When 144 or less, they are easy to find with mental math if you have memorized the multiplication table up to 12s.

88. Draw the two perfect squares with area 4 and 16 from the previous problem.

videos

Video example problems have three images to click on. You can see a video step-by-step answer, a written step-by-step answer, or only the answer. If you find yet more helpful videos, please let your instructor know so that this website can be updated and improved!

BrainSTEM

Metric Conversion Trick!! Part 1

Metric Conversion Trick!! Part 2

YouTube Problems

play video show steps show answer Write as multiplication, then multiply: 32 = 32 = 3 × 3 = 9 9

play video show steps show answer Twelve square feet is how many square inches? One square foot is 12 inches × 12 inches = 144 square inches.
So twelve square feet is twelve of that: 1,728 sq. in.
1,728 sq. in.

play video show steps show answer Three square centimeters is how many square meters? One square meter is 100 cm × 100 cm = 10,000 square cm.
For three square cm we do 3 ÷ 10,000 = 0.0003 sq. m
0.0003 sq. m

play video show steps show answer What is the square root of 225? A square with sides 15 would have area 15 × 15 = 225.
So the answer is 15
15

play video show steps show answer What is the square root of 87? (Round to the thousandths place.) Let our calculator do the work, to find the answer of 9.327 9.327

Textbook cover

Textbook Exercises for Exponents and Square Roots

This list of recommended odd-numbered textbook problems is designed for a hypothetical student with a "typical" math background. If your math foundation is weak, do even more odd-numbered problems. If your math foundation is strong, do fewer.

Please read the advice on doing homework in the study skills page.

Section 1.7 (Page 84) # 11, 13, 15, 19, 21

Section 2.3 (Page 119) # 81, 83

Section 4.6 (Page 247) # 1, 3, 5, 7, 29, 31

Section 7.2 (Page 381) # 1, 3, 11, 15, 29

Ten Exercises

Try these ten exercises on scratch paper. Work in a study group if you can! Notice where your notes need improvement. After you are very happy with your answers, you can use this form to ask me to check your work. Can you get at least 8 out of 10 correct?

1. 1 foot is how many yards?

2. 7.1 miles is how many feet?

3. 5,280 yards is how many miles?

4. An index card is 0.27 mm thick. What is that thickness in centimeters?

5. An index card is 0.27 mm thick. What is that thickness in meters?

6. A football field is 4,844 cm wide. What is that width in kilometers?

7. 80 ounces is how many pounds?

8.5,200 grams is how many kilograms?

9. One gallon is how many fluid ounces?

10. 806 liters is how many milliliters?

Random Exercises

Try these exercises on scratch paper. Work in a study group if you can! Notice where your notes need improvement. Check your work when you are done.

Units and Their History

One Step Conversions

More about SI Prefixes

Exponents and Square Roots